The present invention relates generally to exposure methods, and more particularly to an exposure method used to manufacture devices such as single crystal plates for semiconductor wafers and glass plates for liquid crystal displays (“LCD”). The present invention is suitable, for example, for a projection exposure method for an object with a contact-hole line pattern or a mixture of isolated contact hole and contact-hole line in a photolithography process.
Recent demands on smaller and thinner profile electronic devices have increasingly demanded finer semiconductor devices to be mounted onto these electronic devices. For example, a design rule has attempted to form a circuit pattern of 100 nm or less on a mass production line, and which will expectedly shift to 80 nm or less. The mainstream photolithography technology has conventionally used a projection exposure apparatus that projects and transfers a pattern on a mask (a reticle) onto a wafer.
The following equation provides the resolution R of the projection exposure apparatus using a light-source wavelength λ and a numerical aperture (“NA”) of the projection optical system:                     R        =                              k            1                    ×                      λ            NA                                              (        1        )            
A focus range that may maintain certain imaging performance is called a depth of focus (“DOF”), which is defined in the following equation:                     DOF        =                              k            2                    ×                      λ                          NA              2                                                          (        2        )            
While Equations 1 and 2 indicate that a shorter wavelength and a larger NA are effective to finer processing, it is unfeasible since the DOF disadvantageously decreases in inverse proportion to the NA. In addition, the shorter wavelength would disadvantageously reduce transmittance of a glass material, and a larger NA makes difficult a design and manufacture of a lens.
Accordingly, the resolution enhanced technology (“RET”) has been recently proposed which reduces the process constant k1 for fine processing. One RET is modified illumination, which is also referred to as oblique incidence illumination, multi-pole illumination, or off-axis illumination. The modified illumination arranges an aperture stop with a light-shielding plate on an optical axis in an optical system near an exit surface of a light integrator for forming a uniform surface light source, and introduces exposure light oblique to a mask, as disclosed, for example, in Japanese Laid-Open Patent Application No. 5-21312. The modified illumination may reduce the value of the process constant k1 down to 0.3 smaller than 0.5 in the usual illumination, and contribute to fine processing.
The instant assignee has already proposed another RET, in Japanese Patent Application No. 2002-123268, which illuminates a mask that forms a desired pattern to be transferred and an auxiliary pattern using strong illumination light with high resolving power to resolve the desired pattern, and illumination light with low resolving power to restraining the auxiliary pattern from resolving.
For example, while a mask having only a desired pattern 31 shown in FIG. 13 is usually used to resolve a contact-hole pattern 21 shown in FIG. 12, the instant assignee has proposed that the resolving power improves when a mask pattern is used which arranges not only the desired pattern 31 to form the contact-hole pattern 21 but also an auxiliary pattern 32 smaller than the desired pattern. An exposure method for resolving only the desired pattern 31 by illuminating a mask that includes a desired pattern and a dummy pattern under certain conditions is referred to as “Exposure Method I”. Here, FIG. 12 is a schematic plane view of the contact-hole pattern 21. FIG. 13 is a schematic plane view of a mask pattern that arranges a desired pattern to expose the same.
The mask arranges, like a mask 30 shown in FIG. 14, a desired contact-hole pattern 31 at a predetermined pitch and an auxiliary or dummy pattern 32 around the desired pattern 31. Here, FIG. 14 is a schematic view of the binary mask 30 that forms the desired contact-hole pattern 31 and auxiliary pattern 32. It is preferable that the mask 30 shown in FIG. 1 sets t/d to be 0.7 or greater where “t” is an opening diameter of the desired pattern 31 to resolve the contact-hole pattern 21 and “d” is an opening diameter of the unresolved auxiliary pattern 32 to assist the desired pattern 31 in resolving.
As discussed in detail below, only the desired contact-hole pattern may be exposed with good resolving power onto an object, such as a wafer, by illuminating this mask 30 using plural types of light, such as cross oblique incidence illumination (referred to as enhancement illumination part) to resolve the desired contact-hole pattern, and illumination (referred to as restraint illumination part) to restrain the cross oblique incidence illumination from resolving the auxiliary pattern (in other words, to limit the exposure dose for the auxiliary pattern (a little increased exposure dose) and to enhance the exposure dose for the desired contact-hole pattern (much increased exposure dose)).
When the mask 30 shown in FIG. 14 that has a small pitch in the contact holes is illuminated with small σ illumination, diffracted beams deviate from the pupil surface in the projection optical system in the exposure apparatus except for the 0th order diffracted beam. More specifically, there occur the 0th order diffracted beam 10 and diffracted beams 11-18 of other orders as shown in FIG. 15A, and diffracted beams except for the 0th order diffracted beam deviate from the pupil surface, whereby no pattern is formed under this condition. Here, FIG. 15A is a typical view showing a relationship between diffracted beams and the pupil surface in the projection optical system for small σ illumination to the mask 30 shown in FIG. 14.
Therefore, illumination is required to allow the diffracted beams 11-18 to enter the pupil. For example, in order to allow illustrative two diffracted beams 10 and 15 to enter the pupil in the projection optical system shown in FIG. 15A, the illumination light may be moved to form oblique incidence illumination as shown in FIG. 15B. The oblique incidence illumination may enable the 0th order diffracted beam and one of ±1st order diffracted beams 15 to enter both ends in the pupil in the projection optical system, and interference between these two diffracted beams incident upon the pupil forms linear interference bands at a regular pitch on the object.
FIG. 16 shows a relationship between the 0th order diffracted beam and the 1st order diffracted beams in oblique incidence illumination for a mask pattern with fine pitches. Area “a” of the 0th order diffracted beam, ±1st order diffracted beams occur like “b” and “c”. In FIG. 16, a shape of the area “a” allows one of ±1st order diffracted beams to enter the pupil. Left and right circles of the pupil in the projection optical system have the same diameter as the pupil diameter in the projection optical system, and their centers are offset from the center of the pupil by a predetermined amount (or an interval between 10 and 15 in FIG. 15). In other words, the oblique incidence illumination that arranges the 0th order diffracted beam in the area “a” enables one of ±1st order diffracted beams 15 to enter the pupil, and interference between these two diffracted beams forms linear interference bands at a regular pitch on the object.
Similarly, the oblique incidence illumination that arranges the 0th order diffracted beam in the area “b” enables one of ±1st order diffracted beams 15 to enter the pupil in the area “a”.
As shown in FIG. 17, four streamlined effective light-source areas as a combination area of two circles would form linear infringe bands on an object to be exposed at a regular pitch in longitudinal and lateral directions, and strong and weak parts appear at two-dimensional pitches at intersection points overlapping light intensity distributions. In other words, the above enhancement illumination part corresponds to four (beveled) streamlined areas having a longitudinal direction in a direction perpendicular to a radial direction of crossed pupil as shown in FIG. 17. The other part (i.e., part other than beveled part in the pupil in the projection optical system) corresponds to the above restraint illumination part.
The desired contact-hole pattern 31 on the mask 30 is larger than the auxiliary pattern 32 and thus has stronger light intensity than the peripheral, forming the desired contact-hole pattern. However, as shown in FIGS. 18A and 18B, the mere cross oblique incidence illumination would result in the resolution of the auxiliary pattern and creates an unnecessary pattern other than the desired contact-hole pattern. Here, FIG. 18 is a view of simulated resolved pattern on the object corresponding to the right effective light-source shape.
As shown in FIG. 19, the exposure dose of a thin solid ray resolves not only the desired pattern P1 but also auxiliary pattern P2 when sliced at the exposure-dose threshold or resist threshold shown by a thin dotted line in FIG. 19 where the desired contact-hole pattern P1 has a desired diameter value. Here, FIG. 19 shows a relationship between the exposure dose and an image on the object corresponding to the exposure dose for the crossed oblique incidence illumination and inventive modified illumination.
Accordingly, the instant assignee has proposed to add the effective light-source distribution, i.e., the above restraint illumination part that allows only one diffracted beam to enter the pupil surface, to the effective light-source distribution shown in FIG. 17. Preferably, the diffracted beam is 0th order diffracted light as the only one diffracted beam. FIG. 20 shows an exemplary effective light-source distribution.
The exposure dose shows like a thick solid ray as shown in FIG. 19 when the mask 30 is illuminated using an effective light-source distribution shown in FIG. 21 that has a cross-shaped blank at its center and is close to an addition between the effective light-source distribution shown in FIG. 17 that allows two diffracted beams to enter the pupil and the effective light-source distribution shown in FIG. 20 that enables one diffracted beam to enter the pupil. The exposure dose particularly increases at part corresponding to the desired contact-hole pattern on the mask 30, and provides the desired pattern P3 in which the dummy resolution pattern P2 disappears at the exposure-dose threshold or resist threshold shown by a thick dotted line in FIG. 19 where the desired contact-hole pattern has a desired diameter value.
In summary, Exposure Method I is characterized in using an effective light-source shape 40 shown in FIG. 22 that has two functions with light-transmitting part 41 and light-shielding part 42. In other words, the effective light-source shape 40 may be divided into two, as shown in FIGS. 23A and 23B. Four parts 51 in FIG. 23A serves to effectively resolve the desired pattern, but may possibly resolves the auxiliary pattern 32 around the desired pattern 31. On the other hand, four parts 52 in FIG. 23B clarifies an outline of the desired pattern 31 and serves to restrain the auxiliary pattern 32 from resolving. Therefore, only the desired pattern 31 resolves and the auxiliary pattern 32 does not when the mask 30 shown in FIG. 14 is exposed with an effective light-source shape shown in FIG. 22. Here, FIG. 23 is a plane view that dissolves the effective light-source shape 40 into two, FIG. 23A is a schematic plane view of the part 51 that contribute to resolution of the desired pattern 31, and FIG. 23B is a schematic plane view of the part 52 that restrains the auxiliary pattern 32 from resolving.
The modified illumination needs to create an effective light-source shape corresponding to the modified illumination. One method to form the effective light source for the modified illumination is to insert a stop corresponding to an effective light source formed at a pupil in the projection optical system. However, the stop shields the light and this method has low light use efficiency and lowers throughput as the number of exposed objects per unit time.
Various methods have been proposed to improve light use efficiency in the modified illumination. For example, Japanese Laid-Open Patent Application No. 5-283317 proposes a method to introduce the light using a prism. Use of the method proposed in Japanese Laid-Open Patent Application No. 5-283317 would form an annular effective light source using a cone prism, and a quadrupole effective light source using a pyramid prism.
However, the above RET proposed by the instant assignee needs a sum of an effective light-source distribution as strong modified illumination for improved resolving power and an effective light source distribution to restrain the resolution of the auxiliary pattern from resolving, and this distribution is hard to form in the illumination optical system using a prism.
A method has been proposed, for example, by Japanese Laid-Open Patent Application 4-225514, which forms an effective light source for the modified illumination using a diffractive optical element. While use of computer generated hologram (“CGH”) as a diffraction optical element would form an arbitrary effective light-source shape and the above complex effective light source, the diffraction efficiency of the diffraction optical element is as low as about 80% and improved light use efficiency cannot be expected.